Vaccine Effectiveness Dropped After Delta Became Dominant: US Health Body

The vaccines are still protective, the CDC said, and the finding must be interpreted with caution, as vaccine effectiveness might wane over time and the estimates of efficacy were imprecise.

The effectiveness of Covid-19 vaccines among front-line workers declined to 66% after the delta variant became dominant, compared with 91% before it arose, according to a report from the U.S. Centers for Disease Control and Prevention. The vaccines are still protective, the CDC said, and the finding must be interpreted with caution, as vaccine effectiveness might wane over time and the estimates of efficacy were imprecise. "Although these interim findings suggest a moderate reduction in the effectiveness of Covid-19 vaccines in preventing infection, the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of Covid-19 vaccination," researchers wrote in the agency's Morbidity and Mortality Weekly Report. The findings echo earlier evidence from Israel and the U.K. suggesting Covid vaccines lost some potency in preventing infections over time as the delta variant spread. These and other research results will be under scrutiny next week as CDC advisers weigh the Biden administration's plan to administer booster doses to most vaccine recipients in the U.S.